Klf2 in PC s is required for antibody-mediated protection from influenza infection. (A) Schematic illustration of the experiments to examine the protective ability of serum antibodies against influenza infection. (B and C) Mixed BM chimeras were generated with BM cells from μMT and S1pr2-creERT2 Klf2^+/+ or Klf2^fl/fl mice, vaccinated with inactivated whole PR8 virus, followed by tamoxifen treatment and collection of serum for measurement of anti-HA titers (B) or for transfer into naïve mice (C). (B) High avidity anti-HA IgG responses at the indicated time points after influenza vaccination in S1pr2-creERT2 Klf2^+/+ chimeras (closed circles, n = 10) or S1pr2-creERT2 Klf2^fl/fl chimeras (closed squares, n = 13). (C) Survival of mice that received sera from naïve mice (open circles, n = 6), vaccinated S1pr2-creERT2 Klf2^+/+ chimeras (closed circles, n = 8), or vaccinated S1pr2-creERT2 Klf2^fl/fl chimeras (closed squares, n = 8) before challenge with a high dose of influenza virus. Data in B and C are representative of two independent experiments (n = 10 for each group in B and n = 5 for each group in C, in the second experiment). Data were analyzed by two-tailed unpaired Student’s t test (B) or log-rank (Mantel–Cox) test (C). ns, not significant; PCs, plasma cells; BM, bone marrow. **P < 0.01, ***P < 0.001, and ****P < 0.0001.